a) Field of the Invention
The invention is directed to a shutter, in particular for an objective, for improving the depth of focus in a camera, wherein the shutter has, in a central area, a radially uniform transmission function averaged over the circumferential angle. The invention is further directed to a CCD-camera, especially for endoscopes, in which the shutter can advantageously be used together with the objective.
b) Description of the Related Art
Objectives are known from photographic cameras, for example. In order to limit the light intensity in the exposure of a photographic film, a shutter is normally provided in objectives of this type. While the shutter is usually adjustable in this application and, for that reason, does not appear completely round, this shutter still essentially constitutes a round hole optically, that is, it has a radially independent transmission function in the area of a hole.
However, this shutter has additional functions. First, by limiting the light to rays in the vicinity of the optical axis, the imaging characteristics of the objective are improved by stopping out the outer lens areas. On the other hand, an increased depth of focus is achieved as the shutter decreases in size.
However, the design of this shutter can be problematic particularly in small cameras, for example, CCD-cameras in endoscopes which often require small shutters because of miniaturized construction. A shutter which is too small also has worsened resolving capability for image presentation because, in view of the wavy nature of the light at the edge of the shutter, there is always a noticeable diffraction which limits the possible image resolution and predominates when the shutter is reduced in size below a certain lower limit.
Particularly in the case of CCD-cameras in which the individual elements for image presentation are usually in the order of magnitude of micrometers and with which diffraction effects can also be detected in the micrometer range, a clearly visible diffraction limitation occurs already in shutters in the order of magnitude of millimeters. Naturally, the shutters could be made larger; but there would be a loss in the depth of focus, which is completely unacceptable in the case of endoscopes when the image recording is carried out by means of a CCD-camera.
It is the primary object of the invention to improve known shutters, especially in connection with objectives, so that they are suitable particularly for CCD-cameras and so that the depth of focus as well as the resolution for image presentation can be increased by means of them.
This object is met in a shutter of the type described above in that the transmission function determining the shutter first drops off toward zero in a radial transition area which is a continuation of the central area mentioned above and which has a quantity n of cutouts which extend toward the radial outside and are spaced apart from one another at equal angles 360xc2x0/n in the circumferential direction, and the quantity n of cutouts has the value 5, 6 or 7. It is possible to use a shutter of this kind, according to the invention, in any type of optical beam path, for example, in the beam path of a laser, etc., also without an objective. However, the preferred use of the shutter in connection with an objective is referred to hereinafter.
First of all, it is surprising in terms of physics that this object can be met at all, since the shutter diameter was previously regarded as an insurmountable barrier to improvement of resolution. Heretofore, there has been no suggestion whatsoever for straying from circular shutters with a radial rectangular transmission function, since these were believed to have the best depth of focus and the loss of resolution was always taken for granted with the smaller diameter of the shutter because all of the familiar physics textbooks attributed the maximum attainable resolving capability exclusively to the very small lens diameter or shutter diameter.
Shutters of the type mentioned above always have a sharp edge, so that the diffraction pattern limiting the resolution can be represented mathematically essentially by a zero-order Bessel function. In contrast to this, however, the diffraction pattern of the shutter, according to the invention, for the objective is still a sum of different Bessel functions as a result of the transition area provided according to the invention. Through the selection of the shape of the shutter and/or the configuration of the transmission function, for the purpose of displaying the diffraction pattern, contributions of higher Bessel functions can be added which, because of a different phase relationship, can narrow the broad maximum of the zero-order Bessel function in the neighborhood of small radii. Particularly also due to the area which is provided according to the invention for the diffraction and which is widened by the transition area, the zero-order Bessel function contribution dominating the resolution is also scaled to smaller radii, so that, as a result, the diffraction pattern limiting the resolution is even substantially sharper in the neighborhood of zero. However, because of the higher light intensity in the central area, the depth of focus is essentially entirely determined thereby. Accordingly, with an improved depth of focus, an improved resolution is also possible in particular.
Surprisingly, it has been shown that a camera which is outfitted with a shutter of this type for the objective of the camera is also substantially more sensitive to light. In fact, this could be expected because the central area is somewhat enlarged by the transition area with respect to the transmissivity of the shutter. But at the same time it would have to have been expected that the depth of focus would decrease. However, with a suitable design of the shutter according to the invention, it was observed that the depth of focus did not decrease in spite of increased-light sensitivity.
Due to the cutouts which extend on the radial outside and are spaced at equal angles 360xc2x0/n in the circumferential direction and because of the resulting symmetry, the Bessel functions which additionally contribute to the diffraction and whose solution, as is well known, runs in periodicity with the circumferential angle are selected. Accordingly, this further development forces the occurrence of higher-order Bessel functions than the zero order, especially of an order of the whole number n of 5, 6 or 7, which also enables the selection of a suitable phase position of these contributions based on a suitable shape for a particularly favorable increase in the resolving capability.
Surprisingly, it has proven particularly advantageous when the number n has the value 5, 6 or 7. Since these findings were made through experimentation, a plausible explanation was even found. These Bessel functions have a maximum in the neighborhood of the drop-off of the zero-order Bessel function. When it is made possible, as a result of appropriate shaping of the shutter, to generate these contributions in reverse phase position to the zero-order Bessel function, the total drop-off of the diffraction patterns in the area of the diffraction maximum is substantially steeper and the possible resolution is thus improved in a substantially more effective manner. A shape of this kind is shown later in an embodiment example.
The desired transmission function of the shutter could also be achieved through arrangements of holes in the shutter material in the radial transition area. However, such holes can cause new diffraction effects, so that the resolution by means of the provided transition area remains far from optimum. Accordingly, based on these considerations, a preferable shutter will have no closed cutouts and, above all, no circular cutouts in the transition area forming a continuation of the central area.
Based on these considerations, the shutter was further optimized. According to an advantageous further development in this respect, it has proven particularly favorable when the shutter is formed with a star-shaped transition area around a circular central area.
In particular, the following further developments of the invention have proven especially favorable for the design of the shutter, according to the invention, for an objective. In a first further development of the invention, it is provided in this connection that the radial transmission function drops off to 90% of the central value at a radius R1 and to 10% of the central value at a radius R2, wherein R2xe2x88x92R1 is greater than 0.1 R1.
In another advantageous further development of the invention in this connection, it is provided that the radial transmission function drops off to 90% of the central value at a radius R1 and to 10% of the central value at a radius R2, and (R2xe2x88x92R1) less than R1.
It has been determined in particular that the above-mentioned advantageous results can be achieved in a simple manner when R2/R1 is fixed at 0.8xc2x10.1.
Due to the fact that the radial area continuing the central area is set at the order of magnitude of 10% to 100% of the radius of the central area, the contribution of larger proportions to the increased resolution is not too small to be effective on the one hand and, on the other hand, the continuation of the area up to twice the central area also leads to an approximate doubling of the resolving capability, so that a noticeable increase in resolution can be seen without a substantial worsening of the depth of focus. In particular, the indicated value of R2/R1xe2x88x921=0.8xc2x10.1 has proven particularly suitable for practical applications.
Based on the ideas expressed above which were formed by means of mathematical considerations, such a simple shutter shape would not have been expected. In particular, a shutter of this type can be manufactured in a simple manner. With regard to the further improvement of the shutter according to the invention, this was an additional unexpected result which was particularly advantageous for the use of these shutters.
In another further development of the invention, the shutter is vacuum-deposited on a lens as an opaque layer, especially as a metal layer.
Initially, no special improvement was anticipated with respect to this embodiment form either. However, it has been shown that a substantial increase in resolution can be achieved in this way compared, for example, to an individual shutter which is placed in front of a lens. This is due primarily to the fact that the light passing through the shutter is immediately conducted into the glass of the following lens, so that deviations in the light propagation direction based on Snell""s law of refraction, whereby every change in angle is reduced with respect to the surface normal, are compensated. Therefore, diffraction effects due to proximity to a glass surface resulting from said lens can be kept small.
This advantage becomes apparent above all when the surface normal remains constant in the area of the shutter opening or varies only slightly over the shutter area, namely, when this lens is plane or flat. The above-mentioned advantages can be achieved in an extremely simple manner, for example, in a commercially available CCD-camera, when the shutter is vacuum-deposited on the foremost lens of the objective. The fact that common CCD-cameras with a plane first lens surface are commercially available and the possibility of vacuum-depositing the metal layer on these lenses without completely disassembling the camera advantageously simplifies the required modification in which a shutter according to the invention is used on the objective that is already provided for the camera.
Due to the somewhat elongated optics, it is extremely advantageous for a favorable arrangement of the camera, for endoscopy in particular, to record images from areas which lie at an angle to the optical axis of the CCD-camera. For this purpose, it is provided that a prism and/or an achromat and/or a negative individual lens are/is arranged in front of the shutter located on the front lens. The prism allows a recording of images at an angle to the optical axis of the objective. The achromat is provided, for example, in order to correct for chromatic aberrations caused by the prism.
As stated above, cameras of the type mentioned above can be used in a particularly advantageous manner for endoscopy and the abovementioned embodiment forms for the shutters which will be better understood from the following embodiment forms were optimized especially for the field of endoscopy. However, it has also been shown that the objectives outfitted with a shutter of this kind are also particularly suitable for other CCD-cameras and other objectives which can be used for imaging.